Remove redundant comments in credentials_factory.py

This is a follow-up patch for [1] to fix some nits. It is to remove
redundant comments for get_credentials function, as the comments have
already been included in docstring.

[1] Idd258f00999248a5422876d2d943f1e0595f4a69

Change-Id: I9b17100a86fc24dd575e50f3fcf29ae38ff4d3ec
1 file changed